1. Field of the Invention
The present invention relates to a mechanism for charging a shutter in a camera.
2. Description of the Related Art
Many conventional single-lens reflex cameras use a focal plane shutter. In the focal plane shutter, shutter curtains are charged after a single photographing action. Then, in response to the subsequent pressing of a release button, the force of a driving spring causes the shutter curtains to move from the charged position, thereby allowing a film or an image pickup device to be exposed to light.
There are some conventional mechanisms for charging shutter curtains.
For example, Japanese Patent Laid-Open No. 4-307527 discusses a mechanism that causes a charging member including a cam gear and a cam lever to be driven by the driving force of a motor, thereby charging shutter curtains. To prevent the charging member from blocking the movement of the shutter curtains, this mechanism causes the charging member to move to a pre-exposure retracted position before the movement of the shutter curtains. A second driving mechanism is used to forcibly move the charging member to the pre-exposure retracted position with the power of the motor.
Japanese Patent Laid-Open No. 63-169632 also discusses a mechanism that causes a charging member to be driven by the driving force of a motor, thereby charging shutter curtains. In this mechanism, the charging member is reset to a pre-exposure retracted position by spring force alone.
Conventional shutter charging mechanisms will now be described with reference to FIG. 7 and FIG. 8.
FIG. 7 illustrates an operating state of a cam gear that constitutes a part of a conventional charging mechanism. FIG. 8 illustrates the structure of the cam gear.
FIG. 7 illustrates a driving unit 51 including a motor, a cam lever 52 biased by a spring (not shown) against a cam gear 53, a cam 53a fixed to the cam gear 53, a shaft 52a that rotates while being in contact with the cam 53a, and a contact surface 52b that drives a mirror shutter charging lever (not shown). FIG. 8 illustrates a top dead center section 53b of the cam 53a, a bottom dead center section 53c of the cam 53a, and a lift section 53d of the cam 53a. The top dead center section 53b causes the contact surface 52b of the cam lever 52 to move to the farthest position from the center of the cam gear 53, the bottom dead center section 53c causes the contact surface 52b of the cam lever 52 to move to the closest position to the center of the cam gear 53, and the lift section 53d causes the contact surface 52b of the cam lever 52 to move from the bottom dead center section 53c to the top dead center section 53b. 
FIG. 7 illustrates the position of the cam 53a before camera operation, that is, when a user can view a subject image through a finder. In this state, the shaft 52a of the cam lever 52 is at rest in the front half of the top dead center section 53b. When a shutter is released, the motor in the driving unit 51 causes the cam gear 53 (i.e., the cam 53a) to rotate in the counterclockwise direction in FIG. 7. This counterclockwise rotation causes the shaft 52a of the cam lever 52 to move from the top dead center section 53b to bottom dead center section 53c of the cam 53a. Upon detection of the completion of the movement to the bottom dead center section 53c, the motor stops driving the cam gear 53. A spring force that biases the cam lever 52, which is attached to the mirror shutter charging lever (not shown), in the counterclockwise direction causes the cam lever 52 to move from the top dead center section 53b to the bottom dead center section 53c (i.e., the bottom of the cam 53a) in conjunction with the movement of the cam 53a. That is, the cam lever 52 also rotates counterclockwise in FIG. 7.
When the cam lever 52 rotates counterclockwise, the spring attached to the mirror shutter charging lever (not shown) causes the mirror shutter charging lever to follow the movement of the contact surface 52b. The mirror shutter charging lever causes a mirror (not shown) to move from a viewing position (mirror down position), which allows the viewing of a subject image, to a photographing retracted position (mirror up position), which facilitates photographing. The movement of the mirror shutter charging lever causes a shutter charging lever (not shown) to move from a second position, which allows the charging of a shutter, to a first position (the above-described pre-exposure retracted position), which allows shutter curtains to move.
While the shutter charging lever is at the pre-exposure retracted position, when a leading-curtain driving lever of the shutter and a trailing-curtain driving lever of the shutter are driven, the shutter curtains move to expose a film, an image pickup device, or other imaging system, to light. Upon completion of the exposing operation, the motor in the driving unit 51 causes the cam gear 53 (i.e., the cam 53a) to rotate counterclockwise in FIG. 7 again. This counterclockwise rotation causes the shaft 52a of the cam lever 52 to move over the lift section 53d of the cam 53a and allows the cam lever 52 to rotate clockwise. Then, the mirror shutter charging lever follows the clockwise rotation of the cam lever 52 to cause the mirror to be reset from the photographing retracted position (mirror up position) to the viewing position (mirror down position). The movement of the mirror shutter charging lever drives the shutter charging lever to move from the first position (pre-exposure retracted position), which facilitates the movement of the shutter curtains, to the second position, which facilitates the charging of the shutter. Upon detecting that the shaft 52a of the cam lever 52 has reached the top dead center section 53b of the cam 53a, the motor stops driving the cam 53a. Thus, the mirror is held at the mirror down position while the shutter is held at the charged position.
In general, a focal plane shutter is structured as follows. As described above, the power of a motor is used to drive a shutter charging lever and to cause a cam surface of the shutter charging lever to press shutter curtain driving levers (i.e., the above-described leading-curtain driving lever and trailing-curtain driving lever). This action causes shutter curtains to move to a charged position against the biasing force of a driving spring.
In a charged state, the cam surface of the shutter charging lever supports the shutter curtain driving lever to mechanically hold the shutter curtains at the charged position. Therefore, during the time period in which the camera is not in use, the spring force of the driving spring presses the shutter curtain driving levers against the cam surface of the shutter charging lever.
If the camera remains unused for extended periods of time with the shutter charging lever under the load of the driving spring as described above, a film of lubricating oil over the cam surface of the shutter charging lever becomes dried out. As a result, friction increases the initial actuation load and can interfere with proper operation.
Some measures, such as those discussed in Japanese Patent Laid-Open No. 4-307527 and Japanese Patent Laid-Open No. 63-169632, have been taken to reduce such a problem. For example, to ensure that a shutter charging lever is reliably reset to a pre-exposure retracted position, a second driving mechanism is provided to force the shutter charging lever to the pre-exposure retracted position with the power of the motor. Alternatively, a powerful spring is used to ensure the reset of a shutter charging lever. However, the structure with the second driving mechanism causes large impact, impact noise, and vibrations during operation, while the structure with the powerful spring increases a driving force during charging and can cause significant energy loss.